1. Field of the Invention
The present invention relates to underground structures such as protective grids, a ground structure foundation such as a pile foundation for a pipeline and a pile foundation of a building, etc., pipes buried in a vertical direction such as a water pipe, gas pipe, etc., a drainage channel structure such as a U-shaped ditch, etc. a manhole, an underground storage chamber, an underground storage tank, a basement of a building, etc. and other underground structures (hereinafter simply referred to as "underground structures") constructed in cold regions and, more specifically, a frost heave damage preventive structure of underground structures constructed in a manner so as to protect the underground structure against frost heave and thaw settlement.
2. Background of the Invention
Underground structures constructed in cold regions suffer from damage due to frost heave such as floating and jetting from the surface of the ground, getting broken, etc. by being repeatedly subject to the actions of frost heave and thaw settlement.
The principle of frost heave of underground structures due to this frost heave and thaw settlement will be explained first by taking the protective grid member 1 (hereinafter simply referred to as "protective grid") indicated in FIG. 1 as an example.
As shown in FIG. 1(a), a protective grid 1 buried under the ground (unfrozen soil layer 3) is lifted with frost heave of the soil which is frozen on the side face of the portion included in the frozen soil layer 4 of the protective grid 1 as shown in FIG. 1(b), and the protective grid 1 moves in the unfrozen soil layer 3, as the atmospheric temperature decreases, freezing the soil and causing frost heave. As a result, a cavity 6 is formed under the bottom face of the protective grid 1.
In FIG. 1(b), reference numeral 2' indicates the position of the ground surface before frost heaving of the soil, reference numeral 4 indicates the frozen soil layer, and reference numeral 5 indicates the freezing front (border between unfrozen soil layer 3 and frozen soil layer 4), respectively.
Although the soil around the cavity 6 is not frozen at least at this point in time, the cavity 6 changes in shape and gets smaller under the influence of freezing and thawing action, soil pressure, etc. with the passage of time.
Also, even if the surface of the ground 2 returns to its initial position as the atmospheric temperature increases and the soil settles with thawing, the protective grid 1 cannot return to the original position because of the change in the shape of the cavity 6 and therefore remains heaved on the ground surface 2.
Moreover, this floating accumulates because the protective grid 1 is repeatedly subject to frost action such as frost heave, thaw settlement of frozen soil and, eventually, the protective grid 1 gets in a state in which it protrudes from the surface of the ground 2 as shown in FIG. 1(d). For that reason and also because the amount of frost heave varies from place to place, the protective grid 1 is likely to deform and break. As a result, the protective grid 1 can no longer perform its intended function, which presents a risk of collapse of the face of a slope if combined with other factors such as precipitation, etc.
By the way, it is known that damage by frost heave occurs, not only to underground structures buried in comparatively shallow positions under the ground such as the protective grid 1, but also to underground structures buried in comparatively deep positions under the ground, such as the foundation of a ground structure such as a pile foundation of a pipeline, a pile foundation of a building, etc. The principle of such damage will be explained by reference to the pile foundation 10 (hereinafter referred to simply as "pile" in some cases) indicated in FIG. 4 as an example.
As shown in FIG. 4, a pile 10 buried under the ground is subject to a an upward frost heaving force F produced at the freezing front 5, through the frozen soil layer 4, in a narrow freezing part near the 0.degree. C. isotherm around the pile 10 when the atmospheric temperature decreases, causing freezing and frost heave of the soil. The range of the freezing front 5 in which the frost heaving force acts on the pile 10 (force acting in a way to lift the pile 10) depends on the deforming capacity of the frozen soil layer 4.
On the other hand, forces resisting this frost heaving force are the weight W of the pile itself (dead weight), the weight W of the ground structure (not illustrated) supported by the pile 10, and the frictional force with the unfrozen soil layer 3 around the pile 10.
Also, when the balance between the frost heaving force lifting the pile 10 and the forces resisting this frost heaving force is lost and the frost heaving force lifting the pile 10 becomes larger than the latter, the pile 10 is lifted due to frost heave of the soil, thereby causing substantial damage to the ground structure.
In order to protect underground structures from damage caused by frost heave and thaw settlement, various methods for increasing the frictional force resisting the frost heaving force have been proposed and implemented such as 1) replacing the soil around the underground structure with soil or material not easily frost heaved, 2) lessening the amount of lift due to freezing of soil by increasing the dead weight of the underground structure, 3) increasing the peripheral friction by increasing the buried depth of the underground structure, 4) forming the peripheral face in a special shape such as a wave shape, etc. to increase the friction against upward movement so as to lessen the amount of lift due to freezing of soil, 5) preventing adfreezing of soil by forming a sliding layer or an insulating layer around the underground structure, 6) burying the pile as heat pipe pile to form a frozen soil layer under the pile with the cold heat during the winter season or 7) fixing the pile to the permafrost, etc.
However, these methods had problems such as impossibility of perfectly preventing frost heave of an underground structure, difficulty of obtaining soil or material not easily frost heaved, high costs, restriction to applicable types of underground structures, drop of a protective effect against frost heave damage, etc.